(1) Field of the Invention
The present invention generally relates to image reading technology, and more particularly to technology for reading a document image in a selected one of a plurality of different modes.
(2) Description of the Related Art
Conventionally, copiers capable of two different reading operations are commercially available for office use, for example. One of the modes is to read a document to obtain a color image and the other to obtain a monochrome image. Such copiers generally include a halogen lamp or a fluorescent lamp as a light source for illuminating a document surface. Light reflected from the document surface is received with a CCD sensor, thereby reading an image of the document. The CCD sensor may be a 4-line CCD sensor that is composed of a 1-line CCD sensor dedicated exclusively for monochrome reading (hereinafter, referred to as the “monochrome CCD sensor”) and a 3-line CCD sensor dedicated exclusively for color reading in RGB components (hereinafter referred to as the “color CCD sensor”). Monochrome reading of a document image is carried out with the monochrome CCD sensor, whereas color reading is carried out with the color CCD sensor.
It should be noted, however, color CCD sensors are generally lower in sensitivity than monochrome CCD sensors, and thus the S/N (Signal-to-Noise) ratio of color CCD sensors is lower than that that of monochrome CCD sensors. This leads to a problem that images obtained by color reading and monochrome reading are not equal in terms of image quality.